The machining of a workpiece made from raw stock or material, such as metal, is a form of subtractive manufacturing during which parts of the workpiece is progressively removed until the workpiece reaches an intended shape and size. Operations during machining primarily involve turning, drilling and milling the raw material into a desired part, which respectively require removing the raw material by rotating the raw material against a stationary cutting tool, axially boring holes using a rotating bit with a cutting edge on a distal end whilst the workpiece is held stationary, and cutting away the raw material using a rotating bit with cutting edges generally along the rotating bit's sides and distal end.
Machine tools, or tools, is the parts of machines that came into contact with the workpiece, and achieves turning, cutting, drilling, sanding, knurling, facing, deformation, or other operations. Examples of machine tools includes a drill bit or a milling cutters.
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole of circular cross-section in solid materials. The drill bit is a rotary cutting tool, often multipoint. The bit is pressed against the workpiece and rotated at rates from hundreds to thousands of revolutions per minute (RPM). This forces the cutting edge against the workpiece, cutting off chips from the hole as the workpiece is drilled.
Milling is a cutting process that uses a milling cutter to remove material from the surface of a workpiece. The milling cutter is a rotary cutting tool, often with multiple cutting points. As opposed to drilling, where the tool is advanced along the tool's rotation axis, the cutter in milling is usually moved perpendicular to the rotation axis so that cutting occurs on the circumference of the cutter. As the milling cutter enters the workpiece, the cutting edges of the tool repeatedly cut into and exit from the material, shaving off chips from the workpiece with each pass.
Both drilling and milling involve traversing a rotating bit along a longitudinal axis in the Z-direction. Milling, which operates in three-dimensional space, also involves moving the rotating bit along a plane in X- and Y-directions. The position of a machine tool at a time can be accurately described in the 3-dimentional space. The path a machine tool traversed while processing a workpiece is referred as the tool path, and can be defined by tracking locations on X-, Y-, and Z-axes in a time window. The speed at which the workpiece advances through the cutter is called feed rate, or just feed; which is most often measured in length of workpiece per full revolution of the rotating bit.
Furthermore, there can be more than three axes to define a tool path in certain machines. In a machining process, tools may move in four or more ways to manufacture parts out of metal or other materials by milling away excess material, by water jet cutting or by laser cutting. While a typical computer numerically controlled tools support translation in three axes, multi-axis machines also support rotation around one or more axes. For example, a 5-axis machines may perform a machining process using the linear X-, Y-, and Z-axes and two rotary axes. Typically, there are two types of 5-axis processing. The first is a 3-axis processing with 2-axis indexing, which involves normal linear 3-axis processing after indexing the inclined planes by using the two rotary axes. The second is processing with the two rotary axes while moving with three linear axes.
In machine tools, a spindle is a rotating axis of the machine, which often has a shaft at the center of the axis. The shaft itself is called a spindle; but also, in shop-floor practice, the word often is used metonymically to refer to the entire rotary unit, including not only the shaft itself, but its bearings and anything attached to the shaft. A machine tool may have one or several spindles, such as the headstock and tailstock spindles on a bench lathe. The main spindle is usually the biggest one. References to “the spindle” without further qualification imply the main spindle. Spindles are powered, and impart motion to the machine tools or the workpiece. The spindle speed is the rotational frequency of the spindle of the machine, measured in RPM. The spindle power is the power set to be used by the spindle. The spindle load is the power drawn from the spindle motor. Both the positions on X-, Y-, and Z-axes and spindle speed, spindle power, and spindle load are parameters useful in defining or describing the state and history of the machine tool usage.
A critical factor in securing a successful output of a machining process is the ability of a machine tool to maintain the machine tool's accuracy and repeatability. Changes due to wear or failure of a machine tool can lead to significant losses in production and unexpected downtime. How to detect machine tool wear and tear and to predict a right time for tool maintenance or replacement are a persistent challenge for almost all manufacturing shops.
Monitoring systems are being developed to track operation conditions of machine tools; however, the data collected from sensors are not well correlated with the in-process machining operating conditions, which compromises the prediction accuracy. As a result, the prevailing method of manufacturing shop floor scheduling still relies on a generalized algorithm of time to failure (e.g. exponential fault growth). The method lacks accuracy because the algorithm depends upon typical assumptions rarely applicable in a real machining process. Thus a manufacturing shop without a reliable prediction tool always faces the dilemma of either being over-cautious by unnecessarily disrupting workflow when the tools are still functioning well, or encountering a catastrophic loss for failing to replace a worn tool.
Numerical control (NC) is the automation of machine tools that are operated by precisely programmed commands encoded on a storage medium, as opposed to controlled manually via hand wheels or levers, or mechanically automated via cams alone. Most NC today is computer numerical control (CNC), in which computers play an integral part of the control. The development and proliferation of CNC imposes a higher demand for reliability of machine tools. With the advancement of sensing technology and automation, more information can be collected. However, independently operated machine systems and proprietary interfaces and machine communication protocols can lead to excessive time and cost to utilize the data.
In an effort to facilitate the organized retrieval of process information from numerically controlled machine tools, MTConnect project was initiated. MTConnect is a manufacturing industry standard designed for the exchange of data between shop floor equipment and software applications used for monitoring and data analysis. However, the primary applications developed using MTConnect data are focused on the visualization and reporting of Overall Equipment Effectiveness (OEE) and history of alarms. No insight for assessing and predicting machine tool wear and failure has been reported.
Thus there remains a need for creating a self-aware machine platform in manufacturing shop floor, including detecting machine tool degradation, predict future failure of the machine tool, and provide advice on managing a manufacture floor shop, based on the data collected on the machine tool path and other parameters.